/*
 * kernel/mutex.c
 *
 * Mutexes: blocking mutual exclusion locks
 *
 * Started by Ingo Molnar:
 *
 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
 * David Howells for suggestions and improvements.
 *
 *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
 *    from the -rt tree, where it was originally implemented for rtmutexes
 *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
 *    and Sven Dietrich.
 *
 * Also see Documentation/mutex-design.txt.
 */
#include <seminix/mutex.h>
#include <seminix/spinlock.h>
#include <seminix/tcb.h>
#include <seminix/sched.h>
#include <seminix/sched/rt.h>
#include <seminix/signal.h>
#include <asm/mutex.h>
#include "mutex.h"

/*
 * A negative mutex count indicates that waiters are sleeping waiting for the
 * mutex.
 */
#define	MUTEX_SHOW_NO_WAITER(mutex)	(atomic_read(&(mutex)->count) >= 0)

void
__mutex_init(struct mutex *lock, const char *name)
{
    atomic_set(&lock->count, 1);
    spin_lock_init(&lock->wait_lock);
    INIT_LIST_HEAD(&lock->wait_list);
    mutex_clear_owner(lock);
    lock->spin_mlock = NULL;

    debug_mutex_init(lock, name, key);
}

/*
 * We split the mutex lock/unlock logic into separate fastpath and
 * slowpath functions, to reduce the register pressure on the fastpath.
 * We also put the fastpath first in the kernel image, to make sure the
 * branch is predicted by the CPU as default-untaken.
 */
static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count);

/**
 * mutex_lock - acquire the mutex
 * @lock: the mutex to be acquired
 *
 * Lock the mutex exclusively for this task. If the mutex is not
 * available right now, it will sleep until it can get it.
 *
 * The mutex must later on be released by the same task that
 * acquired it. Recursive locking is not allowed. The task
 * may not exit without first unlocking the mutex. Also, kernel
 * memory where the mutex resides mutex must not be freed with
 * the mutex still locked. The mutex must first be initialized
 * (or statically defined) before it can be locked. memset()-ing
 * the mutex to 0 is not allowed.
 *
 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
 *   checks that will enforce the restrictions and will also do
 *   deadlock debugging. )
 *
 * This function is similar to (but not equivalent to) down().
 */
void __sched mutex_lock(struct mutex *lock)
{
    /*
     * The locking fastpath is the 1->0 transition from
     * 'unlocked' into 'locked' state.
     */
    __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
    mutex_set_owner(lock);
}

/*
 * In order to avoid a stampede of mutex spinners from acquiring the mutex
 * more or less simultaneously, the spinners need to acquire a MCS lock
 * first before spinning on the owner field.
 *
 * We don't inline mspin_lock() so that perf can correctly account for the
 * time spent in this lock function.
 */
struct mspin_node {
    struct mspin_node *next ;
    int		  locked;	/* 1 if lock acquired */
};
#define	MLOCK(mutex)	((struct mspin_node **)&((mutex)->spin_mlock))

static noinline
void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
{
    struct mspin_node *prev;

    /* Init node */
    node->locked = 0;
    node->next   = NULL;

    prev = xchg(lock, node);
    if (likely(prev == NULL)) {
        /* Lock acquired */
        node->locked = 1;
        return;
    }
    ACCESS_ONCE(prev->next) = node;
    smp_wmb();
    /* Wait until the lock holder passes the lock down */
    while (!READ_ONCE(node->locked))
        arch_mutex_cpu_relax();
}

static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
{
    struct mspin_node *next = READ_ONCE(node->next);

    if (likely(!next)) {
        /*
         * Release the lock by setting it to NULL
         */
        if (cmpxchg(lock, node, NULL) == node)
            return;
        /* Wait until the next pointer is set */
        while (!(next = READ_ONCE(node->next)))
            arch_mutex_cpu_relax();
    }
    ACCESS_ONCE(next->locked) = 1;
    smp_wmb();
}

/*
 * Mutex spinning code migrated from kernel/sched/core.c
 */

static inline bool owner_running(struct mutex *lock, struct tcb *owner)
{
    if (lock->owner != owner)
        return false;

    /*
     * Ensure we emit the owner->on_cpu, dereference _after_ checking
     * lock->owner still matches owner, if that fails, owner might
     * point to free()d memory, if it still matches, the rcu_read_lock()
     * ensures the memory stays valid.
     */
    barrier();

    return task_curr(owner);
}

/*
 * Look out! "owner" is an entirely speculative pointer
 * access and not reliable.
 */
static noinline
int mutex_spin_on_owner(struct mutex *lock, struct tcb *owner)
{
    preempt_disable();
    while (owner_running(lock, owner)) {
        if (need_resched())
            break;

        arch_mutex_cpu_relax();
    }
    preempt_enable();

    /*
     * We break out the loop above on need_resched() and when the
     * owner changed, which is a sign for heavy contention. Return
     * success only when lock->owner is NULL.
     */
    return lock->owner == NULL;
}

/*
 * Initial check for entering the mutex spinning loop
 */
static inline int mutex_can_spin_on_owner(struct mutex *lock)
{
    struct tcb *owner;
    int retval = 1;

    preempt_disable();
    owner = READ_ONCE(lock->owner);
    if (owner)
        retval = task_curr(owner);
    preempt_enable();
    /*
     * if lock->owner is not set, the mutex owner may have just acquired
     * it and not set the owner yet or the mutex has been released.
     */
    return retval;
}

static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);

/**
 * mutex_unlock - release the mutex
 * @lock: the mutex to be released
 *
 * Unlock a mutex that has been locked by this task previously.
 *
 * This function must not be used in interrupt context. Unlocking
 * of a not locked mutex is not allowed.
 *
 * This function is similar to (but not equivalent to) up().
 */
void __sched mutex_unlock(struct mutex *lock)
{
    __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
}

/*
 * Lock a mutex (possibly interruptible), slowpath:
 */
static __always_inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, unsigned long ip)
{
    struct tcb *task = current;
    struct mutex_waiter waiter;
    unsigned long flags;
    int ret;

    preempt_disable();

    /*
     * Optimistic spinning.
     *
     * We try to spin for acquisition when we find that there are no
     * pending waiters and the lock owner is currently running on a
     * (different) CPU.
     *
     * The rationale is that if the lock owner is running, it is likely to
     * release the lock soon.
     *
     * Since this needs the lock owner, and this mutex implementation
     * doesn't track the owner atomically in the lock field, we need to
     * track it non-atomically.
     *
     * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
     * to serialize everything.
     *
     * The mutex spinners are queued up using MCS lock so that only one
     * spinner can compete for the mutex. However, if mutex spinning isn't
     * going to happen, there is no point in going through the lock/unlock
     * overhead.
     */
    if (!mutex_can_spin_on_owner(lock))
        goto slowpath;

    for (;;) {
        struct tcb *owner;
        struct mspin_node  node;

        /*
         * If there's an owner, wait for it to either
         * release the lock or go to sleep.
         */
        mspin_lock(MLOCK(lock), &node);
        owner = ACCESS_ONCE(lock->owner);
        if (owner && !mutex_spin_on_owner(lock, owner)) {
            mspin_unlock(MLOCK(lock), &node);
            goto slowpath;
        }

        if ((atomic_read(&lock->count) == 1) &&
            (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {

            mutex_set_owner(lock);
            mspin_unlock(MLOCK(lock), &node);
            preempt_enable();
            return 0;
        }
        mspin_unlock(MLOCK(lock), &node);

        /*
         * When there's no owner, we might have preempted between the
         * owner acquiring the lock and setting the owner field. If
         * we're an RT task that will live-lock because we won't let
         * the owner complete.
         */
        if (!owner && (need_resched() || rt_task(task)))
            goto slowpath;

        /*
         * The cpu_relax() call is a compiler barrier which forces
         * everything in this loop to be re-loaded. We don't need
         * memory barriers as we'll eventually observe the right
         * values at the cost of a few extra spins.
         */
        arch_mutex_cpu_relax();
    }
slowpath:
    spin_lock_mutex(&lock->wait_lock, flags);

    /* once more, can we acquire the lock? */
    if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
        goto skip_wait;

    debug_mutex_lock_common(lock, &waiter);
    debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));

    /* add waiting tasks to the end of the waitqueue (FIFO): */
    list_add_tail(&waiter.list, &lock->wait_list);
    waiter.task = task;

    for (;;) {
        /*
         * Lets try to take the lock again - this is needed even if
         * we get here for the first time (shortly after failing to
         * acquire the lock), to make sure that we get a wakeup once
         * it's unlocked. Later on, if we sleep, this is the
         * operation that gives us the lock. We xchg it to -1, so
         * that when we release the lock, we properly wake up the
         * other waiters:
         */
        if (MUTEX_SHOW_NO_WAITER(lock) &&
            (atomic_xchg(&lock->count, -1) == 1))
            break;

        /*
         * got a signal? (This code gets eliminated in the
         * TASK_UNINTERRUPTIBLE case.)
         */
        if (unlikely(signal_pending_state(state, task))) {
            ret = -EINTR;
            goto err;
        }

        __set_task_state(task, state);

        /* didn't get the lock, go to sleep: */
        spin_unlock_mutex(&lock->wait_lock, flags);
        schedule_preempt_disabled();
        spin_lock_mutex(&lock->wait_lock, flags);
    }
    mutex_remove_waiter(lock, &waiter, current_thread_info());
    /* set it to 0 if there are no waiters left: */
    if (likely(list_empty(&lock->wait_list)))
        atomic_set(&lock->count, 0);
    debug_mutex_free_waiter(&waiter);

skip_wait:
    mutex_set_owner(lock);

    spin_unlock_mutex(&lock->wait_lock, flags);
    preempt_enable();
    return 0;

err:
    mutex_remove_waiter(lock, &waiter, task_thread_info(task));
    spin_unlock_mutex(&lock->wait_lock, flags);
    debug_mutex_free_waiter(&waiter);
    preempt_enable();
    return ret;
}

/*
 * Release the lock, slowpath:
 */
static inline void
__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);
    unsigned long flags;

    spin_lock_mutex(&lock->wait_lock, flags);
    debug_mutex_unlock(lock);

    /*
     * some architectures leave the lock unlocked in the fastpath failure
     * case, others need to leave it locked. In the later case we have to
     * unlock it here
     */
    if (__mutex_slowpath_needs_to_unlock())
        atomic_set(&lock->count, 1);

    if (!list_empty(&lock->wait_list)) {
        /* get the first entry from the wait-list: */
        struct mutex_waiter *waiter =
                list_entry(lock->wait_list.next,
                       struct mutex_waiter, list);

        debug_mutex_wake_waiter(lock, waiter);

        wake_up_process(waiter->task);
    }

    spin_unlock_mutex(&lock->wait_lock, flags);
}

/*
 * Release the lock, slowpath:
 */
static __used noinline void
__mutex_unlock_slowpath(atomic_t *lock_count)
{
    __mutex_unlock_common_slowpath(lock_count, 1);
}

static noinline int __sched
__mutex_lock_interruptible_slowpath(struct mutex *lock);

/**
 * mutex_lock_interruptible - acquire the mutex, interruptible
 * @lock: the mutex to be acquired
 *
 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 * been acquired or sleep until the mutex becomes available. If a
 * signal arrives while waiting for the lock then this function
 * returns -EINTR.
 *
 * This function is similar to (but not equivalent to) down_interruptible().
 */
int __sched mutex_lock_interruptible(struct mutex *lock)
{
    int ret;

    ret =  __mutex_fastpath_lock_retval(&lock->count);
    if (likely(!ret)) {
        mutex_set_owner(lock);
        return 0;
    } else
        return __mutex_lock_interruptible_slowpath(lock);
}

static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);

    __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_);
}

static noinline int __sched
__mutex_lock_interruptible_slowpath(struct mutex *lock)
{
    return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, _RET_IP_);
}

/*
 * Spinlock based trylock, we take the spinlock and check whether we
 * can get the lock:
 */
static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);
    unsigned long flags;
    int prev;

    spin_lock_mutex(&lock->wait_lock, flags);

    prev = atomic_xchg(&lock->count, -1);
    if (likely(prev == 1)) {
        mutex_set_owner(lock);
    }

    /* Set it back to 0 if there are no waiters: */
    if (likely(list_empty(&lock->wait_list)))
        atomic_set(&lock->count, 0);

    spin_unlock_mutex(&lock->wait_lock, flags);

    return prev == 1;
}

/**
 * mutex_trylock - try to acquire the mutex, without waiting
 * @lock: the mutex to be acquired
 *
 * Try to acquire the mutex atomically. Returns 1 if the mutex
 * has been acquired successfully, and 0 on contention.
 *
 * NOTE: this function follows the spin_trylock() convention, so
 * it is negated from the down_trylock() return values! Be careful
 * about this when converting semaphore users to mutexes.
 *
 * This function must not be used in interrupt context. The
 * mutex must be released by the same task that acquired it.
 */
int __sched mutex_trylock(struct mutex *lock)
{
    int ret;

    ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
    if (ret)
        mutex_set_owner(lock);

    return ret;
}

/**
 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
 * @cnt: the atomic which we are to dec
 * @lock: the mutex to return holding if we dec to 0
 *
 * return true and hold lock if we dec to 0, return false otherwise
 */
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
{
    /* dec if we can't possibly hit 0 */
    if (atomic_add_unless(cnt, -1, 1))
        return 0;
    /* we might hit 0, so take the lock */
    mutex_lock(lock);
    if (!atomic_dec_and_test(cnt)) {
        /* when we actually did the dec, we didn't hit 0 */
        mutex_unlock(lock);
        return 0;
    }
    /* we hit 0, and we hold the lock */
    return 1;
}
